The present invention relates to an under thread supply apparatus for a sewing machine and a method of automatically supplying an under thread to a sewing machine.
In a conventional lock stitch sewing machine, when an under thread or a bobbin thread wound on a bobbin is used up, the bobbin, together with a bobbin case, is taken out of a shuttle. After wound by another under thread, the bobbin is set to the shuttle. This sequence of operations is manually performed. The manual work takes much time and much labor. To cope with this, various types of automatic devices for the thread winding and the bobbin exchange have been proposed. For the automatic devices, reference is made to Published Unexamined Japanese Patent Application Nos. Sho. 61-168388 and 61-172589, and Hei. 1-91897.
The device disclosed in Published Unexamined Japanese Patent Application No. Sho. 61-168388 detects the reduction of the under thread on the bobbin in the shuttle by a leftover thread detector, and automatically exchanges the bobbin with the leftover thread with a new bobbin with an under thread already wound therearound.
The device disclosed in Published Unexamined Japanese Patent Application No. Sho. 61-172589 automatically supplies the under thread to a plural number of sewing machines. Each of the sewing machines is provided with a drive device for driving a bobbin case pull-out mechanism, a bobbin case set-in mechanism, and an automatic under thread supply device (or a bobbin thread feeding device) per se. When an under thread exchange instruction is issued, the drive device is driven to move the automatic under thread supply device to a related sewing machine, a bobbin case accommodating a bobbin with a preset amount of under thread wound therearound is set in the shuttle, while a bobbin with the consumed under thread is pushed out of the shuttle.
In the device disclosed in Published Unexamined Japanese Patent Application No. Hei. 1-91897, an under thread guided by a guide shaft is nipped between it and a bobbin. It is brought into engagement with the rotation center of the bobbin. A take-up shaft is turned by a second drive device, so that the bobbin engaging the take-up shaft is turned. In this way, the under thread is wound around the bobbin automatically.
Those proposed devices have the following problems.
The amount of an under thread to be wound on a bobbin is set at a fixed value. Therefore, a considerably large amount of the thread is left not used for some types of yarn and some types of stitching patterns. Further, a detection of the remaining under thread for providing a reference for the exchange of the bobbin in the shuttle is rough made. Because of this, the leftover thread is inevitably created.
In the device of Published Unexamined Japanese Patent Application No. Hei. 1-91897, the under thread is wound around the bobbin still in the shuttle. Accordingly, the stitching operation by the sewing machine is interrupted during this winding. The efficiency of the sewing operation is deteriorated. The remaining devices each have not such a construction that the under thread is wound around the bobbin still in the shuttle. Those devices are free from the problem of deteriorating the sewing operation efficiency. The description of the publication does not refer to the timing of the bobbin exchange. Therefore, there is a danger that a mutual interference of the bobbin with the sewing machine takes place.
In the conventional bobbin exchanging devices of the under thread supply apparatus, a shuttle, a bobbin, and a bobbin case are constructed to have special shapes. Delicate interrelations among those components greatly influence the stitching quality. Accumulation of long time experiences creates a good stitching quality. Less maintenance lessens economical loss. For these reasons, a less modification is required for those three components (shuttle, bobbin, and bobbin case).
Accordingly, an object of the present invention is to provide an automatic under thread supply device which produces a reduced amount of the leftover thread and is economical, and ensures a smooth bobbin exchange without any mutual interference of the bobbin with a sewing machine.
Another, an object of the present invention is to provide a leftover thread removal device for a bobbin in use with a automatic under thread supply device in which related components and members are laid out without creating any restriction, and a leftover thread can be removed certainly.
Still another object of the present invention is to provide a bobbin exchanging device for a sewing machine which allows a shuttle, a bobbin case, and a bobbin to be used without any modification thereof, and keeps the stitching quality good.